LCD devices are widely used as display devices in computers, televisions and other information processing devices. In particular, LCDs of TFT type (hereinafter also referred to as “TFT-LCDs”) are widely used and their market is expected to keep on growing, with an accompanying demand for ever greater image quality. The explanation below deals with a TFT-LCD example, but the present invention is not limited to TFT-LCDs, and is also applicable to passive matrix type LCDs, plasma-address LCDs and the like, and is generally applicable to all LCDs that perform display on the basis of an LC that is sandwiched between a pair of substrates having respective electrodes formed thereon, wherein voltage is applied across the electrodes.
To date, the most widely used mode in TFT-LCDs is the so-called TN mode, in which an LC having positive dielectric anisotropy is aligned horizontally between mutually opposing substrates. In TN-mode LCD devices, the alignment direction of the LC molecules adjacent to one of the substrates is twisted by 90 degrees with respect to the alignment direction of the LC molecules that are adjacent to the other substrate. Such TN-mode LCD devices are manufactured in accordance with an established manufacturing technology that is inexpensive and industrially mature, although there remains room for improvement in that high contrast is difficult to realize.
Meanwhile, so-called VA LCD devices have also been disclosed in which an LC having negative dielectric anisotropy is aligned vertically between mutually opposing substrates (for instance, Patent Document 1). As disclosed in Patent Document 1, for instance, in a VA (vertical alignment) LCD device, the LC molecules are aligned in a direction substantially vertical to the substrate surface when no voltage is applied, and hence the LC cell exhibits virtually neither birefringence nor optical rotation, and light passes through the LC cell without hardly any change in the polarization state thereof. Therefore, substantially complete black display can be realized, when no voltage is applied, by arranging a pair of linear polarizers with the LC cell therebetween in such a manner that the absorption axes thereof are substantially orthogonal to each other. When a voltage is applied, the LC molecules become tilted substantially parallel to the substrates, and exhibit as a result large birefringence that translates into white display. Therefore, such VA LCD devices can easily realize very high contrast, which is not possible in TN mode.
However, the VA LCD device having the configuration described above has room for improvement in that the viewing angle cannot be widened easily. VA LCD devices afford substantially complete black display, since an LC cell exhibits virtually no birefringence in frontal view, and two polarizers are completely orthogonal to each other, as described above. The LC cell, however, exhibits birefringence and has apparent retardation at an oblique viewing angle. Also, the geometric relative relationship of the two polarizers is no longer apparent orthogonality, and hence light leakage occurs, which results in lower contrast and narrower viewing angle. In VA LCD devices, therefore, a retardation film is often arranged with a view to canceling the extra retardation of the LC cell in the oblique viewing angle, and with a view to maintaining the orthogonality of the cross-Nicol arranged polarizers, at the oblique viewing angle. For example, conventional techniques disclosed for widening the viewing angle involve arranging polarizers on both sides of a VA LC cell, and arranging, between the polarizer and the LC cell, at least one retardation film from among a uniaxial retardation film (so-called positive A plate) having an in-plane optic axis and wherein extraordinary index>ordinary index, a uniaxial retardation film (so-called negative C plate) having an out-of-plane (film normal direction) optic axis and wherein extraordinary index<ordinary index, or a biaxial retardation film, (for instance, Patent Documents 1 to 4).
To achieve a wider viewing angle in a VA LCD device, as described above, it is important to (1) maintain orthogonality of the polarizers in a cross-Nicol arrangement at the oblique viewing angle, similar to that in frontal view, and (2) cancel the extra retardation of the LC cell at the oblique viewing angle. Conventionally, (1) and (2) are achieved by arranging appropriate retardation films. This approach for widening the viewing angle through the use of retardation films is widely known. In all conventional techniques, however, the design of the retardation conditions is optimized for a single wavelength alone (ordinarily around 550 nm), and thus light leakage occurs upon black display at wavelengths other than the design wavelength. Therefore, there is room for improvement as regards the occurrence of coloring at oblique viewing angles.
In order to solve the above problems, the inventors filed an earlier patent application (for instance, see Patent Document 5), on the basis of the finding that the above problem can be solved by using reverse wavelength dispersion retardation films (reverse wavelength dispersion-type retardation films) in which there are completely separated, in terms of wavelength dispersion characteristic, (1) preservation of the orthogonality of polarizers disposed in a cross-Nicol arrangement at an oblique viewing angle, in the same way as in frontal view, during black display; and (2) cancellation of the extra retardation in the LC cell at an oblique viewing angle, such that (1) and (2) are compensated for each different retardation film in the LCD device.
[Patent Document 1]
Japanese Kokai Publication No. 2000-39610
[Patent Document 2]
Japanese Kokai Publication No. Hei-11-258605
[Patent Document 3]
Japanese Kokai Publication No. Hei-10-153802
[Patent Document 4]
Japanese Kokai Publication No. 2000-131693
[Patent Document 5]
WO 06/001448